1. Field of the Invention
The present invention relates to the use of soda lime glass for making fluorescent lamps. It also relates to a glass with strong UV-absorbing properties, which at the same time has reduced absorption in the visible range and to a process for making it.
2. Related Art
Known glasses with UV-absorbing properties are usually used for making liquid crystal displays (LCD), monitors and/or display screens, as well as for making gas discharge tubes, and especially fluorescent lights. This sort of glass is used for light sources, among other things, in rear illuminated display screens (so-called backlight displays). For this sort of application the fluorescent lights must have only very small dimensions and correspondingly the lamp glass has only an extremely small thickness.
The gas acting as a light source contained in this sort of lamp is ignited by applying a voltage by means of electrodes, i.e. to the light source. Usually the electrodes are arranged within the lamp, i.e. the electrically conducting metal wires pass through the lamp glass and are sealed in the lamp glass in a gas-tight manner. However it is also possible to ignite the gaseous light source and/or the plasma inside the lamp by an externally generated electric field, i.e. by electrodes that are outside of the lamp, which do not pass through the lamp glass. This sort of lamp is generally known as an EEFL lamp (external electrode fluorescent lamp). In this sort of lamp it is important that the radiated high frequency energy is not absorbed or only absorbed to a small extent by the lamp glass, in order to ignite the gas acting as the light source in the fluorescent lamp. This assumes that the glass has an extremely small dielectric constant and an extremely low dielectric loss factor, tan δ. The dielectric loss factor acts as a measure for the energy absorbed in the excited dielectric alternating field and converted into dissipated heat.
Furthermore the permeability or transmission of visible light should be kept relatively constant in glasses used for this sort of application up to the wavelength range below 400 nm, especially below 380 nm.
In contrast to the high light transmission in the visible range the transmission in the UV range should be as small as possible or the glass should be as impermeable as possible for light in the UV range.
Gas discharge tubes, especially fluorescent lamps, emit a large fraction of their radiation in the UV range, which has a damaging effect for the surrounding components, such as polymer and other plastic, so that they become yellowish and/or brittle, which can make the entire product unusable. Also turbidity, a so-called haze, can be produced in optical components exposed to intense UV radiation. The mercury line at 313 nm is an especially damaging emission line. Thus one goal is to make glass of this type, which absorbs this emission line as completely as possible. Thus it is desirable to change the high transmission in the visible range to a more or less complete absorption within a few nanometers at the transition to the UV range, which is also called a sharp UV cutoff. The less the spacing between maximum transmission and maximum absorption, the steeper is the UV cutoff.
Fluorescent lamp glasses for the above-described application, which absorb UV radiation to the desired extent, are known from U.S. Pat. No. 5,747,399. However it has been shown that this sort of glass is characterized by a strong discoloration in the visible range and strong solarization. Frequently a yellow brown discoloration is produced already during melting of the raw materials.
Strongly UV-absorbing glass, which is suitable as a lamp glass for fluorescent lights, especially those used for light sources for liquid crystal displays (LCD) in rear-illuminated displays, is known from 10 2004 027 119.4. This glass has good transmission in the visible range from 400 to 800 nm besides outstanding absorption behavior for UV light up to 335 nm and is fused or bonded with metals, especially electrode metals guided through it, such as Fe, Co and Ni alloys (e.g. KOVAR® alloys) and with tungsten and/or molybdenum metals.
A zirconium oxide-containing and lithium oxide-containing borosilicate glass of high resistance is known from DE-A 198 42 942, which is especially suitable for use as solder or fusing glass with Fe—Co—Ni alloys. One such glass can also contain color-imparting ingredients, such as Fe2O3, Cr2O3, CoO and TiO2.
U.S. Pat. No. 4,565,791 describes a glass for ophthalmologic applications, which has special refractive indices and Abbé numbers and densities suitable for that purpose. A glass of this sort has a UV-absorption limit between 310 and 335 nm and contains TiO2 as UV absorber. It is explicitly stated that refining with chloride is necessary in many cases in order to make this glass, since As2O3 and Sb2O3 refining is insufficient. Finally the reference likewise states that although the glass of this type is extremely thin a combination of Fe2O3 and TiO2 leads to discoloration of the glass, which is the reason that quartz material with an iron content of less than 100 ppm should be exclusively used.